1. Field of the Invention
The invention relates to a transconductance tuning circuit and related method thereof, and more particularly, to a transconductance tuning circuit utilizing an amplitude lock loop to tune the transconductance and related method thereof.
2. Description of the Prior Art
In today's information society, documents, data, video, and audio information can be transferred through electronic signals. Therefore, the signal processing circuits associated with the transferring of the electronic signals have become the most importance hardware in the modern society.
The signal processing circuit often includes various kinds of filters. As is well known, one technique for implementing a filter is to utilize a transconductor cell (also known as a Gm cell) to build a transfer function of a filter. The above-mentioned transconductor cell is a circuit capable of transforming an input voltage into an output current according to a transconductance. That is, the transformation relationship (e.g., ratio) between the input voltage and the output current is determined by the transconductance of the transconductor cell. Furthermore, a transconductor cell can cooperate with a capacitor such that an integrator can be achieved. Furthermore, by combining a plurality of integrators, the filter can achieve different kinds of transfer functions. For example, by combining two transconductor cells and two capacitors, a first-order transfer function can be achieved. By combining four transconductor cells and four capacitors, a two-order transfer function can be achieved. Because the above-mentioned filter is composed of transconductor cells and capacitors, the filter can be called as transconductor-capacitor (Gm-C) filter. The Gm-C filter can perform a high-frequency and continuous-time filtering function. Therefore, the Gm-C filter becomes a new research point.
Obviously, the characteristic of the above-mentioned Gm-C filter is determined by the capacitance of each capacitor and the transconductance of each transconductor cell. For example, the bandwidth of the filter, the poles, and the zeros of the transfer function, the gain of the filter are all related to the transconductance and the capacitance. In addition, the ratio of the transconductance and the capacitance (Gm/C) is one of the most important parameters because the ratio often dominates the frequency response of the filter (e.g., it dominates the poles and zeros of the above-mentioned transfer function). However, due to the manufacturing variance and operational temperature variance, the transconductance and the capacitance may shift such that the ratio changes. This directly influences the characteristic of the filter.
To compensate for the non-ideal factors such as the above-mentioned manufacturing and temperature variances, tunable transconductor cells are utilized in a normal Gm-C filter. Furthermore, a transconductance tuning circuit is utilized to adjust the transconductance of each transconductor cell to compensate for the non-ideal factors. That is, because the transconductor cells inside the filter are tunable, the transconductance tuning circuit can properly adjust the transconductance such that the characteristic (e.g., the ratio Gm/C) of the filter can be adjusted to the original design characteristic. For example, if the capacitance of each capacitor inside the filter increases 5% due to the manufacturing process variances, the transconductance tuning circuit can adjusts the transconductance of each transconductor cell to make the transconductance increase 5%. Therefore, the ratio Gm/C is compensated to achieve the original value such that the characteristic integrity of the filter can be maintained.
But, the prior art transconductance tuning circuit and technique often occupies a significant layout area and consumes significant power because the transconductance tuning circuit is often very complex. This makes the entire Gm-C filter, including the transconductance tuning circuit, require a larger layout area, cost more to manufacture, and consume more power. Obviously, these problems concerning the Gm-C filter require solutions.